1. Field of the Invention
Cold seal cohesives are generally natural or synthetic rubber latex compounds, which, when applied to a flexible packaging substrate, allow the package to be cohesively sealed by pressure and dwell time at temperatures from ambient to a temperature of less than about 150.degree. F. Cold seal cohesives are particularly useful in wrapping products which are heat sensitive, such as confectioneries. Cold seal cohesives are usually applied in a pattern on to the inside (cold seal receptive surface) of a film lamination. Cold seal cohesives may also be applied to a single web film. In each case the outer film surface (facing away from the cold seal) must repel adhesion of the cold seal cohesive on the inner film surface in order that the typical manner of supplying film, i.e., in roll stock form, may be easily unwound when it is desired to use the film to wrap the product.
The present invention is directed to films of either the single web or lamination form which has improved properties for packaging with cold seal cohesives.
2. Description of the Related Art
In the case of the prior art single web films, the surface of the film opposing the cold seal receptive surface is typically coated with an overlacquer, e.g., a polyamide, in order to provide sufficient release of that surface from the cold seal cohesive, i.e., to prevent what is known in the art as "roll blocking".
In the case where laminated films are used for packaging, the outer web film is generally modified with migratory additives, which, when sufficiently bloomed to the surface, promote low coefficient of friction (C.O.F.) (for machinability) and cold seal release (CSR). It is known from experience that certain migratory additives, particularly amides, coming in intimate contact with the cold seal surface or cohesive, in sufficient quantities, will cause the cold seal cohesive strength to become significantly weakened. This phenomenon is known in the trade as cold seal deadening.
It is also known that biaxially oriented polypropylene (BOPP) film, by itself, does not give adequate cold seal release (CSR) or coefficient of friction (C.O.F.) and requires the addition of additives to accomplish these objectives. These additives, which are predominantly migratory, have two main problems:
(1) They must bloom to the film's surface and remain there in order to be consistently effective; and, PA1 (2) They have a tendency to retard the cold seal cohesive strength. PA1 conventional linear low density polyethylene (LLDPE) PA1 ethylene-butene plastomer (formed by a metallocene catalyst) PA1 syndiotactic homopolymer of polypropylene (formed by a metallocene catalyst) PA1 homopolymer of high density polyethylene (HDPE)
The current industry standard (state of the art) film for packaging applications is a monolayer BOPP (homopolymer) amide modified film from AET (Applied Extrusion Technology, Inc.) designated B523 or B522. In the case of B523, the slip modified film is corona discharge treated to an equal level on both surfaces and as such can be used on either side. In the case of B522, the film is corona discharge treated on only one surface. The untreated surface is the preferred surface for cold seal release (CSR).
These films need to be aged for a period of time after production thereof, at a certain temperature, so that the CSR and C.O.F. additives sufficiently bloom to the surface for the film to be functional.
In my copending application Ser. No. 08/224,229, filed Apr. 7, 1994, I disclosed the discovery of good to excellent cold seal release in a film without the use of migratory additives or overlacquers. The preferred method disclosed therein was a coextruded skin layer of a polymer blend of ethylene-propylene random copolymer (RCP) and ethylene-butylene (EB) copolymer in the ratio of 60:40 wt %. The skin layer thereof contained crosslinked silicone and Kaolin clay as C.O.F. enhancers. The cold seal release surface was preferably a non-treated, i.e., no corona discharge or other conventional treatments, surface.
The key component of the polymeric blend release system in my previous application was an ethylene-butylene random copolymer in an amount of 20 to 50 wt % of the skin layer where the ethylene content in the ethylene-butylene random copolymer was at least 0.5 wt %.
The present invention concerns cold seal release surfaces which do not contain polybutylene, migratory additives, nor overlacquers.